U.S. Pat. No. 4,557,739 to Fortman et al is assigned to the assignee of the present application, and is incorporated by reference into this specification. It describes a baffle precleaner which may be advantageously used in a sweeper having a vacuumized dust control system. This precleaner has been applied in a number of different sweeper models, where it has successfully fulfilled the objective of reducing dust load going to the related air filters. This has permitted extending the time between filter cleanings, which has reduced the wear and tear on the filters and the mechanisms for cleaning them. Thus the precleaner has proven to be a useful device, and it seems assured of a place in future generations of sweepers.
However, several years of field experience with this precleaner have revealed a problem that arises in its operation. The device is comprised of metal sheets with slotted openings in them through which the air stream passes. These slots are large enough to pass small pieces of solid debris as well as dust. For example, broken fragments of dry leaves, tiny scraps of paper, bits of thread or string and similar small items will pass through the precleaner. They lodge on the filters, and when these are cleaned such debris items fall to the bottom of the filter box and mix with the dust that is also dislodged from the filters.
This mixture of dust and debris accumulates and periodically must be emptied. The precleaner is often applied in a high dump sweeper which is dumped by tilting the hopper and filter box forward and emptying them through a dump door in the front of the hopper, as described in connection with FIG. 1 of the '739 patent. In this configuration, during the dumping operation the dust in the filter box slides forward along the bottom of the box and falls through the slots in the precleaner, after which it mixes with debris from the hopper and passes out through the dump door in the front of the hopper. Pure dust will do this without any trouble. However, when small debris items as described above are mixed with the dust, they tend to bridge across the precleaner slots and obstruct the dust from falling through the precleaner. This has made it necessary to manually clean out such debris, which is a dirty, time-consuming job. There is a need for a better way of emptying dust and debris from the filter box of such sweepers.